Tubular mould for continuous casting

ABSTRACT

In the continuous casting of round or polygonal billet and bloom formats, use is made of moulds the mould cavity of which comprises a copper tube ( 3 ) which is intensively cooled by means of water-circulation cooling. In order to increase the cooling capacity on the one hand and the dimensional stability of the mould cavity ( 4 ) on the other hand, and also extend the total service life of the copper tube ( 3 ), it is proposed to provide the copper tube ( 3 ) with a supporting shell ( 12 ) or supporting plates over the entire circumference at the tube outer lateral surface ( 5 ). For the cooling of the copper tube ( 3 ), cooling ducts ( 6 ) for guiding the cooling water are arranged on the copper tube ( 3 ) or on the supporting shell ( 12 ). The cooling ducts ( 6 ) are distributed over the entire circumference at the tube outer lateral surface ( 5 ) and extend substantially over the entire mould length.

CROSS REFERENCE TO PRIOR APPLICATION

This is a U.S. national phase application under 35 U.S.C. §371 ofInternational Patent Application No. PCT/EP2004/003712, filed Apr. 7,2004, and claims benefit of European Patent Application No. 03008681.3,filed Apr. 16, 2003, which is incorporated by reference herein. TheInternational Application was published in German on Oct. 28, 2004 as WO2004/091826 A1 under PCT Article 21(2).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a tubular mould for the continuous casting ofround and polygonal billet and bloom cross-sections according to theprecharacterising clause of claim 1 or 2.

2. Description of Related Art

In the continuous casting of steel in billet and small bloomcross-sections, tubular moulds are used. Such tubular moulds comprise acopper tube fitted into a water jacket. In order to achieve circulationcooling with a high flow rate of the cooling water, a tubular displaceris arranged outside the copper tube with a small gap relative to thecopper tube. The cooling water is forced through between the displacerand the copper tube over the entire circumference of the copper tube ata high pressure and high flow rate of up to 10 m/s and above. To preventany damaging deformations of the copper tube during casting operationdue to the high temperature differences between the mould cavity sideand the cooling water side, the copper tubes, which are essentially heldonly at the lower and upper tube end by flanges, must have a minimumwall thickness. This minimum wall thickness is dependent on the castingformat and is between 8 and 15 mm.

Since the beginning of industrial continuous casting, efforts have beenmade by those skilled in the art to increase the casting speed in orderto achieve higher outputs per strand. The increase of the castingcapacity is closely related to the cooling capacity of the mould. Thecooling capacity of a mould wall or of the entire mould cavity isinfluenced by many factors. Important factors are the thermalconductivity of the copper tube, the wall thickness of the mould wall,the dimensional stability of the mould cavity in order to avoiddistortion or air gaps between the strand skin and the mould wall, etc.

However, besides the cooling capacity, which may exert a directinfluence on the output per strand for a given strand format, theservice life of the mould also constitutes an important cost factor forthe economic efficiency of the continuous casting plant. The servicelife of a mould expresses how many tonnes of steel can be cast into amould before wear phenomena in the mould cavity, such as abrasive wear,material damage, in particular hot cracks, or damaging deformations ofthe mould cavity, necessitate a change of mould. Depending on the stateof wear, the mould tube has to be scrapped or undergo refinishing sothat it can be used again. In the case of standard conical moulds,moulds with somewhat greater copper tube wall thicknesses have higherdimensional stabilities.

SUMMARY OF THE INVENTION

The object of the invention is to provide a continuous casting mould forbillet and bloom formats which affords, in particular, a higher coolingcapacity and hence allows higher casting speeds, without reaching thelimits of thermal loadability of the copper material. Furthermore, thismould is to have a higher dimensional stability during casting operationand hence produce less abrasive wear as the strand skin passes throughthe mould on the one hand and a more uniform cooling or better strandquality on the other hand. In particular, formation of diamond-shapedstrand cross-sections is to be avoided. In addition, the mould is toachieve an extended total service life and hence reduce the mould costsper tonne of steel.

This object is achieved according to the invention by the characterisingfeatures of claim 1 or 2.

The following advantages can be obtained on continuous casting with thetubular mould according to the invention. The lower wall thickness ofthe copper tube compared with the prior art ensures a higher coolingcapacity with a corresponding increase in the output of the continuouscasting plant. The supporting plates arranged substantially over theentire circumference stabilise the geometry of the mould cavity againstdistortion of the thermally loaded copper walls of the mould tube, sothat on the one hand the mould wear is reduced and on the other hand thestrand quality is improved, as a result in particular of a more uniformcooling. An extended service life is obtained through reduced thermalloading of the copper material and lower abrasive wear between thestrand skin and the mould walls. The total service life is however alsoextended through refinishing operations in the mould cavity, such asre-copperplating of worn spots with subsequent remachining etc., thecopper tube remaining connected to the supporting shell or to thesupporting plates during these operations. In the case of machining,this facilitates the clamping, and when milling or planing etc.vibrations of the copper tube are prevented by the supporting plates,thereby allowing higher machining speeds together with a highdimensional accuracy of the mould cavity. The fact that the supportingplates remain on the copper tube during the reconditioning of the coppertube also reduces, however, the work required to demount thewater-circulation cooling arrangement of the mould, thereby reducingreconditioning costs.

The cooling ducts can be partially let into or milled into thesupporting plates and into the outer lateral surface of the copper tube.To increase the contact area between the copper tube and cooling medium,it is advantageous for the cooling ducts to reduce the wall thickness ofthe copper tube in the region of the cooling ducts by about 30-50%.

If the cooling ducts at the tube lateral surface are milled into thecopper tube, supporting and connecting ribs can be arranged between thecooling ducts without significantly reducing the cooling capacity.According to an exemplary embodiment, it is proposed that the coolingducts take up 65%-95%, preferably 70%-80%, of the outer surface of thecopper tube. Depending on the cross-section of the mould cavity, theresidual wall thickness of the copper tube in the region of the coolingducts is set at about 4 mm to 10 mm. By a suitable choice of thecooling-duct geometry and/or cooling-duct coating, the heat transmissionto the cooling water can be set in accordance with the localrequirements.

In the case of rectangular strand formats, four supporting plates arereleasably or fixedly attached to the copper tube. In order to ensurethat the supporting plates bear against the copper tube in a manner freefrom play irrespective of the manufacturing tolerances, according to anexemplary embodiment the supporting plates can on the one hand butt attheir end face against and on the other hand overlap their neighbouringplates. Neighbouring supporting plates are screwed together in thecorner regions of the copper tube and thus form a supporting boxarranged around the copper tube.

Depending on the design used for clamping the copper tube, thesupporting plates can clamp the copper tube without play and rigidly, orin the case of polygonal formats small gaps for seals, preferablyelastic seals, can be provided between the individual supporting platesat the overlaps. Such small gaps can take up thermal expansion of thecopper tube walls and/or dimensional tolerances of the copper tubelateral surface.

Depending on the extent of the thermal and mechanical loading of theinner wall of the mould cavity by liquid steel or a thin strand skin, orby a predetermined strand skin deformation inside the mould cavity,supporting and connecting ribs which support the copper tube on thesupporting plates or on the supporting shell and/or connect it theretoare to be arranged accordingly.

According to an exemplary embodiment, at the lateral surface of thecopper tube, for each side of the strand, narrow supporting surfaces arearranged along the corner regions and depending on the format one or twoconnecting ribs are arranged in the middle region of the strand sides,the connecting ribs being provided with securing devices to preventmovements transversely to the strand axis. Such securing devices cancomprise, for example, a dovetail profile, a T-profile for slidingblocks or generally a positive or non-positive securing device. As thesupporting plates are advantageously not removed during a reconditioningof the mould cavity, soldered and adhesively-bonded joints can also beemployed.

In the case of moulds with a curved mould cavity, the two supportingplates which support the curved side walls of the mould areadvantageously provided with plane outer sides, to enable the mould tobe clamped without distortion onto a table of a finishing machine duringthe refinishing.

A suitable material for the supporting plates is, for example,commercial quality steel, provided that the mould is not equipped withan electromagnetic stirring device. The compact construction of thecopper tube with its supporting plates and cooling ducts lyingtherebetween facilitates the use of electromagnetic stirring devices.Further advantages of electromagnetic stirring devices can be obtainedthrough the choice of material of the supporting plates. According to anexemplary embodiment, the supporting plates or the supporting shell canbe fabricated from a metallic material (austenitic steel etc.) ornon-metallic material (plastic etc.) which can be easily penetrated by amagnetic field. Composite materials may also be included in the choiceof materials.

According to a further exemplary embodiment, it is proposed to arrangeelectromagnetic coils outside the supporting plates or the supportingshell, or to fit movable permanent magnets into the supporting plates orthe supporting shell.

If the supporting plates are produced from a metallic material, it isadvantageous to prevent the electrolytic corrosion due the cooling waterby a protective layer arranged between the supporting plates and thecopper tube. Such a protective layer can be constructed, for example, bya copperplating of the supporting plate. It is however also possible toclose off the cooling ducts let into the copper tube with a copper layerproduced by electrodeposition.

The cooling ducts in the copper tube are connected to water supply anddischarge lines at the supporting plates or at the supporting shell.According to an exemplary embodiment, it is advantageous for the watersupply and discharge lines to be arranged alongside each other on thesupporting plates at the upper end of the mould and to be connectable tothe cooling-water system by means of a quick coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained below withreference to figures, in which:

FIG. 1 shows a longitudinal section through a mould according to theinvention for round strands,

FIG. 2 shows a horizontal section along the line II-II in FIG. 1,

FIG. 3 shows a longitudinal section through a curved mould for a squarebillet cross-section,

FIG. 4 shows a horizontal section along the line IV-IV in FIG. 3,

FIG. 5 shows a partial horizontal section through a mould corner,

FIG. 6 shows a vertical section through a further example of a mould,and

FIG. 7 shows a partial horizontal section through a mould corner of afurther exemplary embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

In FIGS. 1 and 2, a continuous casting mould for round billet or bloomstrands is depicted by 2. A copper tube 3 forms a mould cavity 4.Provided at the outer side of the copper tube 3, which side forms thetube outer lateral surface 5, is water-circulation cooling for thecopper tube 3. This water-circulation cooling comprises cooling ducts 6distributed over the entire circumference and substantially over theentire length of the copper tube 3. The individual cooling ducts 6 aredelimited by supporting and connecting ribs 8 and 9, respectively, anadditional task of which is to guide the cooling-water circulation intothe cooling ducts 6 from a water supply line 10 to a water dischargeline 11. 12 depicts a supporting shell which surrounds the copper tube 3over the entire circumference and over the entire length and supportsthe copper tube 3 at the tube outer lateral surface 5 via the supportingribs 8. The connecting ribs 9 connect the copper tube 3 to thesupporting shell 12. The supporting shell 12 forms with its innerlateral surface the outer boundary of the cooling ducts 6.

The cooling ducts 6 are let into the outer lateral surface of the coppertube 3 and thereby reduce the wall thickness of the copper tube 3 by20%-70%, preferably by 30%-50%, compared with the copper-tube thicknessat the supporting ribs 8. The thinner the wall thickness of the coppertube 3 in the region of the cooling ducts 6 can be made, the greater theheat transmission from the strand to the cooling water becomes, while atthe same time the operating temperature of the copper wall during thecasting is also reduced. Lower operating temperatures in the copper wallnot only reduce the distortion of the mould tube 3 but also the wear,such as for example cracks in the bath surface region or abrasive wearin the lower mould region.

14 in FIG. 1 schematically depicts a stirring coil for stirring theliquid crater during the continuous casting in the mould. It is clearlyevident that, through the compact construction of the mould and with itsreduced copper wall thickness, the stirring coil 14 is very closelyadjacent to the mould cavity 4 and hence magnetic field losses arereduced compared with conventional moulds. In magnetic fieldapplications, supporting plates or the supporting shell 12 are producedfrom a metallic material which can be easily penetrated by magneticfields, preferably from stainless austenitic steel. It is however alsopossible to produce the supporting shell 12 or supporting plates fromnon-metallic materials, for example from carbon laminate etc.

In FIGS. 3 and 4, a mould for square or polygonal billet and bloomstrands is depicted by 20. A curved copper tube 23 forms a curved mouldcavity 24 for a circular arc-type continuous casting machine.Water-circulation cooling is provided between the copper tube 23 andsupporting plates 32-32′″. Supporting and connecting ribs 28 and 29,respectively, are provided in cooling ducts 26. The water-circulationcooling is essentially designed the same as that described in FIGS. 1and 2. Instead of the tubular supporting shell 12 in FIGS. 1 and 2, thecopper tube 23 in FIGS. 3 and 4 is clamped between four supportingplates 32-32′″ which form a supporting box. The supporting plates32-32′″ are connected to the copper tube 23 via the connecting ribs 29,and the outer lateral surface 25 of the copper tube 23 can be supportedon the supporting plates 32-32′″ at supporting ribs 28. The foursupporting plates 32-32′″ are screwed together, to form a rigid boxaround the copper tube 23, in such a way that each supporting plate32-32′″ butts at its end face against one neighbouring plate andoverlaps the other neighbouring plate. Symbols 34 indicate screws orother connecting elements. The supporting plates 32-32′″ can bereleasably connected to the copper tube 23 by, for example, dovetail orsliding-block guides, clamping screws, threaded bolts etc. It is howeveralso possible to connect the copper tube 23 to the supporting plates 32or the supporting shell 12 (FIGS. 1+2) by soldered or adhesively bondedjoints etc., since for refinishing of the copper tube 23, such aselectrolytic re-copperplating and subsequent machining, the copper tube23 remains connected to the supporting plates 32 or the supporting shell12.

At four corner regions 35 with supporting ribs 28′, the copper tube 23is clamped or supported on the box of the supporting plates 32-32′″. Thecopper tube 23 is generally produced by cold drawing and has in thecorner regions and at the supporting ribs 28, 28′ the wall thicknessresulting from the production process. This wall thickness isessentially dependent on the strand format to be cast and is generally11 mm for a strand format of 120×120 mm² and 16 mm for 200×200 mm². Thecooling ducts 6, 26 are made, by milling, in such a way that apredetermined water circulation between a cooling-water inlet openingand a cooling-water outlet opening is ensured. In the region of thecooling ducts, the copper tube 23 has a residual wall thickness of 4-10mm. The cooling ducts 6, 26 take up an area of 65%-95%, preferably70%-80%, of the outer surface (tube lateral surface 25) of the coppertube 23. The narrow supporting surfaces 28′ on both sides of the fourtube corners contribute considerably to maintaining the geometry of themould cavity. They ensure that the four angles of the copper tube 23 donot become distorted during the casting operation. The risk of producingdiamond-shaped strands is thereby partially eliminated.

Between the corner regions there are provided connecting ribs 29 whichconnect the copper tube 23 to the supporting plates 32-32′″ via securingdevices. They ensure that bending of the copper tube walls towards themould cavity 24 or lateral displacement transversely to the strandrunning direction can be avoided. Known positive and non-positiveconnections are possible as the securing devices, such as for exampledovetail profiles or T-profiles for sliding blocks, welded-on bolts etc.

In the case of curved moulds, it is advantageous for the two supportingplates 32, 32″ which support the curved side walls of the copper tube 23to have plane boundary surfaces 36, 36″ at their sides opposite thecurved supporting surfaces.

In FIG. 5, a supporting plate 51 overlaps a supporting plate 52, whichbutts at its end face 53 against the supporting plate 51. Arrangedbetween the two plates 51, 52 is an elastic seal 54 which, besides thesealing function to prevent cooling water from escaping, can take upsmall tolerances in the external dimensions of the copper tube, but alsosmall expansions of the copper tube wall transversely to the strandwithdrawal direction.

In order to eliminate electrolytic corrosion between the cooling ducts55 of the copper mould 56 and the supporting plates 51, 52, thesupporting plates 51, 52 can be covered with a protective layer 57 ofcopper or an electrically non-conducting layer. As an alternative to aprotective layer 57, the cooling ducts 55′ for example can be closedoff, after being milled into the copper wall, with an electrodepositedcopper layer 58.

59 in FIG. 5 depicts a connecting rib which is fixedly connected to thesupporting plate by soldering or adhesive bonding.

In FIG. 6, an example of water-circulation cooling in cooling ducts 61,61′ along an outer lateral surface 62 of a copper tube 63 is depicted.Cooling water is supplied to the cooling ducts 61 through a pipe system64 outside supporting plates 65. In the lower part 66 of the mould, thecooling water is deflected by 180° and led to the cooling ducts 61′. Thecooling water is discharged from the mould via a pipe system 68. 67schematically depicts a coupling plate which couples or uncouples thepipe systems 64, 68 to or from a water supply when the mould is set downon a mould table (not depicted).

As examples of further measuring points 69, temperature sensors fittedin the outer lateral surface 62 of the copper tube 63 are indicated,these sensors measuring the temperatures at various locations on thecopper tube 63 during the casting operation. Such measurements can beused to graphically represent a temperature profile of the entire coppertube 63 on a screen.

The cooling ducts 61′, which are let into the copper wall and whichreturn the cooling water and lead it to the pipe system 68, can also berun as closed return ducts in the supporting plates 65. In such anarrangement, the heating of the cooling water and the copper walltemperatures can be further reduced.

The cooling ducts in FIGS. 1-6 can be let into the copper tube byvarious production processes. It is possible to mill the cooling ductsinto the outer or inner lateral surface of the copper tube andsubsequently close them off with an electrodeposited layer. To furtherincrease the wear resistance in the mould cavity, hard chromium plating,which is known in the prior art, can be provided in the mould cavity.

In FIG. 7, cooling ducts 71 are arranged in supporting plates 72, 72′. Acopper tube 70 is chosen which is very thin in terms of its wallthickness, for example 3 mm-8 mm. Accordingly, such thin copper tubes 70are frequently supported by supporting surfaces 74 formed on thesupporting plates 72, 72′. Fastening surfaces 77 or connecting profiles78 are generally provided on the copper tube 70. The copper tube 70 isreleasably or fixedly connected to the supporting plates 72, 72′ byfastening devices, such as for example a connecting bolt 75 or adovetail-profile plate 76 with one or more tie rods 79.

1. Mold for the continuous casting of round billet and bloom formats,comprising a copper tube, which forms a mold cavity, and an arrangementfor cooling the copper tube by water-circulation cooling, wherein thecopper tube is provided over its entire circumference and substantiallyover its entire length with a supporting shell which supports the coppertube at its outer lateral surface at supporting surfaces thereof, thesupporting surfaces comprising at least supporting ribs and connectingribs provided with securing devices to prevent transverse movements ofthe copper tube, and further comprising cooling ducts delimited by thesupporting ribs and the connecting ribs arranged for guiding the waterdistributed over the entire circumference and substantially over theentire mold length in one of the copper tube and the supporting shell,wherein said supporting shell comprises grooves for receiving saidconnecting ribs.
 2. Mold according to claim 1, wherein the cooling ductsreduce wall thickness of the copper tube where the cooling ducts arelocated by an amount selected from the group consisting of 20% to 70%and 30% to 50%.
 3. Mold according to claim 1, wherein the cooling ductsoccupy an area of the outer surface of the copper tube selected from thegroup consisting of 65% to 95% and 70% to 80%.
 4. Mold according toclaim 1, wherein the copper tube has a residual wall thickness of 4 mmto 10 mm where the cooling ducts are located.
 5. Mold according to claim1, wherein the cooling ducts are milled into the copper tube and areclosed off with a copper layer produced by electrodeposition.
 6. Moldaccording to claim 1, wherein the supporting shell consists of amaterial selected from the group consisting of a metallic material,austenitic steel, and non-metallic material which can be easilypenetrated by magnetic fields.
 7. Mold according to claim 1, furthercomprising externally-arranged magnetic devices selected from the groupconsisting of electromagnetic coils and moving permanent magnets. 8.Mold according to claim 1, further comprising a protective layer toprevent electrolytic corrosion arranged between the supporting shell andthe copper tube.
 9. Mold according to claim 1, further comprisingcooling-water supply lines and discharge lines arranged at an upper endof the mold that can be connected to cooling-water by a coupling plate.10. Mold according to claim 1, wherein the securing devices are selectedfrom the group consisting of a dovetail profile, a T-profile for slidingblocks and a clamping device.
 11. Mold according to claim 1, wherein thesecuring devices permit longitudinal movements of the copper tube. 12.Mold for the continuous casting of polygonal billet and bloom formats,comprising a copper tube, which forms a mold cavity, and an arrangementfor cooling the copper tube by water-circulation cooling, wherein thecopper tube is provided at the tube outer lateral surface, substantiallyover the entire circumference and substantially over the entire length,with supporting plates which are connected to the copper tube and whichsupport the walls of the copper tube at supporting surfaces thereof, thesupporting surfaces comprising at least supporting ribs and connectingribs provided with securing devices to prevent transverse movements ofthe copper tube, and further comprising cooling ducts delimited by thesupporting ribs and the connecting ribs arranged for guiding the waterdistributed over the entire circumference and substantially over theentire mold length in one of the copper tube and the supporting plates,wherein said supporting shell comprises grooves for receiving saidconnecting ribs.
 13. Mold according to claim 12, wherein in the case ofthe mold is rectangular and four supporting plates are releasablyattached to the copper tube, each supporting plate butting at an endface against an adjacent plate and overlapping another adjacent plate.14. Mold according to claim 12, wherein supporting plates adjacent toeach other are screwed together in corner regions of the copper tube andform a supporting box arranged around the copper tube.
 15. Moldaccording to claim 12, further comprising elastic seals which allowexpansions of the copper tube walls arranged in overlap gaps between thesupporting plates.
 16. Mold according to claim 12, further comprisingnarrow supporting surfaces arranged along corner regions thereof andwherein at least one of the supporting ribs or the connecting ribs arearranged in a middle region of the mold sides.
 17. Mold according toclaim 12, wherein the securing devices are selected from the groupconsisting of a dovetail profile, a T-profile for sliding blocks and aclamping device.
 18. Mold according to claim 12, wherein the copper tubeforms a curved mold cavity and has curved supporting surfaces and thesupporting plates have surfaces opposite the curved supporting surfaces.19. Mold according to claim 12, wherein the securing devices permitlongitudinal movements of the copper tube.